The present invention relates to a solid-state image sensor, in particular, it relates to a solid-state image sensor being provided with an effective pixel area which performs photoelectric conversion in taking in rays of light and an optical black area which forms a reference black level.
Examples of a solid-state image sensor includes a CCD (Charge Coupled Device) image sensor, a CMOS image sensor, etc. A CCD image sensor 1 as shown in FIG. 3 is provided with an effective pixel area 2 having a light receiving portion which performs photoelectric conversion by taking in light and an optical black area 3 which forms a reference black level without taking in light.
In the effective pixel area 2, a plurality of effective pixels (not shown in a drawing), each having a light receiving portion, are disposed in vertical and horizontal directions, similarly, in the optical black area 3, a plurality of optical black pixels are disposed in vertical and horizontal directions.
An effective pixel in the effective pixel area 2 is composed of a light receiving portion 11 which performs photoelectric conversion and is formed on the surface layer portion of a silicon substrate 10 as shown in FIG. 5A. A transfer register 13 is formed on one side of the light receiving portion 11 through a read gate 12 and a channel stop 14 is formed on the other side of it. A transfer electrode 16 is formed on the silicon substrate 10 covering a part right above the transfer register 13 through an insulating film 15 and has an opening port in a part right above the light receiving portion 11. The opening port part of the transfer electrode 16 forms an electrode opening port portion 16a having a rectangular shape as shown in FIG. 6.
Further an inter-layer insulating film 17 is formed covering the transfer electrode 16 and a light shielding film 18 made of aluminum (Al) is formed over the inter-layer film 17 on the silicon substrate 10 as shown in FIG. 5A. The light shielding film 18 is provided with an opening port 19 which opens right above part of the light receiving portion 11, that is, in in communication with the electrode opening port 16a. Light is led in through the opening port 19 and the light is input onto the light receiving portion 11. And other light is shielded from coming in.
On the other hand, the optical black pixels in the optical black area 3 have almost the same constitution as that of the effective pixels in the effective pixel area 2. However, they are different from the effective pixels only in that they do not have opening ports 19 in the light shielding film 18 as shown in FIG. 5B (Refer to FIG. 6.). Otherwise, the effective pixels and the optical black pixels have the same construction including the electrode opening port portion 16a not being covered with the transfer electrode 16 and also the transfer register 13, the channel stop 14, etc. being incidental to the transfer electrode 16, which means that the effective pixels and the optical black pixels have the same unit cell size.
The reason why the electrode opening port 16a is also made in the optical black area 3 is that in the total pixel area 4 (FIG. 3) composed of the effective pixel area 2 and the optical black area 3, before the patterning is performed to form the opening port 19 on the shielding film 18, the effective pixel area 2 and the optical black area 3 are not separately processed and they are processed in the same way in order to simplify the manufacturing process. As described in the above, before the patterning of the light shielding film 18 is performed, the effective pixel area 2 and the optical area 3 are processed in the same way. So that when the patterning for forming the transfer electrode 16, is performed the same sized electrode opening port portions 16a are formed in both the effective pixel area 2 and the optical black area 3.
In recent years, solid-state image sensors such as CCD image sensors have been improved in order to respond to the demand for higher sensitivity, higher degree of integration and further miniaturization. However, nowadays, the requests for the miniaturization, etc. exceeds the speed of improvement, and further new improvements are demanded.
In the past, as a technique to respond to the demand for improving the degree of integration, the technique of making the light shielding film thinner was proposed, and is used in a part of actual production. However, it can be considered that the thickness of the film may become so thin that transmission of light sometimes occurs because of the dispersion in manufacturing. In such a case, since the electrode opening port portion 16a and the light receiving portion 11 are also formed in the optical black area 3 in the same way as in the case of the effective pixel area 2, photoelectric conversion may occur in the light receiving portion 11 in the optical black area 3, which may alter the reference black level and cause a malfunction.
The present invention was invented in consideration of the above-mentioned circumstances, and the object of the present invention is to offer a solid-state image sensor in which higher sensitivity, higher degree of integration and further miniaturization are possible and also the reference black level can be kept in a stable state.
A solid-state image sensor according to the present invention includes an effective pixel area having a light receiving portion which performs photoelectric conversion by taking in light and an optical black area which provides a reference black level without taking in light. The light receiving portion is formed in an electrode opening port portion which is not covered by a transfer electrode. An electrode opening port portion which is not covered by the transfer electrode in the optical black area is formed smaller than that in the effective pixel area as a means to solve the above-mentioned problem.
In the case of a solid-state image sensor of this type, because the electrode opening port portion formed in the optical black area is made smaller than that in the effective pixel area, a unit cell size of the optical black pixels is made smaller than the unit cell size of the effective pixels. Therefore, when the solid-state image sensor of this type is manufactured, theoretical yield of chips obtained from a wafer is increased and the optical black area is narrowed down because of the smaller size of a unit cell. Thus the effective pixel area can be increased corresponding to the narrowed down area in the optical black area, which makes it possible to upgrade the sensitivity and the degree of integration.
Further, since the electrode opening port portion of the optical black area is formed narrow, even when the transmission of light occurs with the tendency of thinning the light shielding film, the quantity of incoming light is small. Therefore, it is possible to control the quantity of photoelectric conversion in the optical black area to be sufficiently.
A solid-state image sensor according to the present invention may also be provided with an effective pixel area having a light receiving portion which performs photoelectric conversion by taking in light and an optical black area which forms a reference black level without taking in light, in which the light receiving portion is formed inside the electrode opening port portion which is not covered with a transfer electrode and an electrode opening port portion which is not covered with the transfer electrode is not formed in the optical black area. This will be a means for solving the above-mentioned problem.
In the case of a solid-state image sensor of this type, because an electrode opening port portion is not formed in the optical black area, a unit cell size of an optical black pixel becomes smaller than that of an effective pixel. Therefore, for example, when a solid-state image sensor of this type is manufactured, the theoretical yield of chips obtained from a wafer is increased and also the area used for the effective pixel area can be increased in proportion to the decreased area of the optical black area due to the down sizing of a unit cell size of an optical black pixel, which makes it possible to obtain a means to upgrade the sensitivity and the degree of integration.
Furthermore, an electrode opening port portion is not formed in the optical black area, accordingly there is no light receiving portion being incidental to the electrode opening port portion, so that even when the transmission of light through a shielding film occurs due to the use of a thin light shielding film, the photoelectric conversion in the optical black area can be completely suppressed.